In communications systems using digital modulation schemes and transmitting information through a communications channel, a conversion of the information from a binary or digital format to an analogue signal is typically required. The information is thus transmitted through the communication channel by for example a signal at a predetermined carrier frequency which is modulated according to the information being transmitted. Any reliably detectable change in or value of a physical parameter (e.g., amplitude, phase, frequency, etc.) of the signal sent by a transmitter of the communications system may reflect the information and, to retrieve the information, the change or value may be detected and demodulated by a receiver of the communications system.
The conversion of the information from the digital format to the analogue signal is typically performed in the transmitter in a converter unit that converts a digital value into a specific value out of a discrete set of M preset values (M being a positive integer), hereinafter referred to as the modulation signal set, for a modulated parameter, for example the phase, the amplitude or the frequency of the modulated signal.
Preceding the conversion, the digital value may be obtained e.g., by forming a symbol of N bits from a binary sequence in a manner that each symbol has a value out of a set of M discrete numbers (M=2N). It is typical that this conversion is performed by first mapping the digital value to a complex symbol or complex value with a real part and an imaginary part out of a set of M complex values, hereinafter referred to as the constellation set, using some predefined mapping scheme. The real part and the imaginary part are then used to define respectively an in-phase component and a quadrature component, i.e., the component with 90° degree phase shift with respect to the in-phase component, of the modulated signal transmitting the information. Thus, each point in the constellation set corresponds one to one to a single point in the modulation signal set.
The complex symbol or complex value converted by the converter unit may be represented in a so called constellation diagram showing the real part of the complex symbol or complex value in the abscissa and the imaginary part of the complex symbol or number in the ordinate. An M-order digital modulation has M complex symbols or complex values. M-order digital modulations of different types also have different constellation diagrams.
U.S. Pat. No. 7,916,808 B2 discloses a modulation circuit that uses pre-calculated and stored data to generate a modulated output. The pre-calculated data are Gaussian filtered sine and cosine responses that are stored in a look-up table of a ROM (Read-only memory). Table 2 at page 3 of U.S. Pat. No. 7,916,808 B2 shows this look-up table that associates for any of the eight theoretical possible complex values of an 8PSK digital modulation plus eight extra complex values rotated of 3π/8 with respected to the theoretical complex values, an in-phase cosine component and a quadrature sine component. The modulated output is then calculated as a sum of values read from the look-up table stored in the ROM.
U.S. Pat. No. 7,236,543 B2 discloses a modulation method and an apparatus for an 8PSK modulation. The modulation method disclosed in U.S. Pat. No. 7,236,543 B2 comprises the steps of mapping the input symbols, generating look-up table addresses, looking-up the table and processing the data, and accumulating, summing and outputting the modulated signals.
In any of the mentioned prior art documents a look-up table is used that associates with indexes or addresses stored in a memory all digital values formed by all possible theoretical binary sequences of the 8PSK digital modulation to the corresponding complex values in the constellation diagram. In other words, the conversion of the digital value, formed by the binary sequence, into the complex value is provided off-the-shelf in a memory with a look-up table.